Polishing slurry

ABSTRACT

A polishing slurry for use in chemical mechanical polishing is disclosed. The polishing slurry contains a solvent and polishing particles dispersed in this solvent. The polishing particles are selected from silicon nitride, silicon carbide, and graphite. The primary particle size of the polishing particles dispersed in the solvent is appropriately 0.01 to 1000 nm. When the polishing particles are colloidally dispersed in the solvent, the secondary particle size of the polishing particles is appropriately 60 to 300 nm.

This is a continuation of application Ser. No. 08/747,382, filed Nov.12, 1996, now U.S. Pat. No. 5,861,054, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing slurry used in thefabrication of semiconductor devices and, more particularly, to apolishing slurry used in CMP (Chemical Mechanical Polishing).

2. Description of the Related Art

Semiconductor devices such as ICs and LSIs are typically fabricatedthrough the following steps: an integrated circuit designing step ofdesigning integrated circuits; a photomask formation step of forming aphotomask used in a lithography step; a wafer manufacturing step ofmanufacturing wafers having a predetermined thickness from asingle-crystal ingot; a wafer processing step of forming a plurality ofintegrated circuits on each wafer; a dicing step of dicing each of theintegrated circuits formed on the wafer into the shape of asemiconductor chip; an assembly step of packaging the dicedsemiconductor chips; and a testing step of testing the packagedsemiconductor chips. Of these steps, the wafer processing step is mostimportant. The wafer processing step is further subdivided into a thinfilm deposition step of depositing a thin film, a lithography step ofexposing/developing a photoresist, an etching step of etching a wafer orthe deposited thin film, and an ion implantation step of implanting animpurity ion into the wafer or the deposited thin film. These steps aredone by using semiconductor fabrication apparatuses dedicated to therespective steps.

The techniques used in the etching step are roughly classified into twocategories.

One is selective etching in which the target surface is masked using aphotoresist and only selected portions are etched. This etching is usedin the patterning of interconnections and the formation of contactholes.

The other is etch back by which a whole wafer is evenly etched. Thisetch back is performed to planarize the wafer surface roughened byinterconnections or planarize the wafer surface after trenches orrecesses are buried with a thin film. As a method of etch back, a methodof performing RIE (Reactive Ion Etching) on the wafer surface afterrecesses on the wafer surface are buried with a photoresist is used mostoften (this method will be referred to as etch back-RIE hereinafter).Unfortunately, this etch back-RIE has some disadvantages that the methodrequires a step of coating the target surface with a photoresist,damages easily remain on the wafer surface after etch back, a dangerousetching gas is used in an RIE apparatus, and the global planarization ofan entire wafer is rather low due to variations of the etching rate onthe wafer surface.

In consideration of these drawbacks, CMP (Chemical Mechanical Polishing)is recently beginning to be studied in place of etch back-RIE.

In CMP, the surface to be polished of a wafer is pressed against apolishing pad adhered to a polishing disc, and the wafer and thepolishing disc are rotated while a polishing agent is supplied to thepolishing pad, thereby polishing the surface to be polished. Thepolishing agent used in CMP is a liquid prepared by dispersing polishingparticles which mechanically polish the surface to be polished in apolishing solution which chemically etches the surface. This liquidpolishing agent has a function of setting the surface to be polished inan active state in which the surface is readily chemically polished,thereby assisting the mechanical polishing by the polishing particles.This liquid polishing agent is called a slurry.

CMP can alleviate some problems of etch back-RIE. However, although CMPcan achieve high global planarization, the local planarization obtainedby CMP is found to be low in fine portions of a semiconductor devicestructure. When the wafer surface planarized by using CMP is observed,fine micron-order dish-like recesses called "dishing" are found in fineportions of a semiconductor device structure, particularly in portionsmade from different substances on the wafer surface.

A typical condition in which "dishing" occurs will be described belowwith reference to FIGS. 1A to 1D.

FIGS. 1A to 1D are sectional views showing trench isolation steps inorder.

FIG. 1A shows the state in which trenches 5 are formed in a siliconsubstrate 1. A polishing stopper film 2 is formed on the surface of thesilicon substrate 1 except the portions in which the trenches 5 areformed. This stopper film 2 is a nitride film (Si₃ N₄).

Subsequently, as shown in FIG. 1B, silicon dioxide (SiO₂) is depositedinside the trenches 5 and on the stopper film 2, forming an oxide film6. The trenches 5 are buried with the oxide film 6.

In FIG. 1C, CMP is performed for the oxide film 6. As a consequence, thesurface of the oxide film 6 dishes to form "dishing" 7.

In FIG. 1D, the stopper film 2 is removed.

In trench isolation in which the "dishing" 7 is formed as shown in FIGS.1C and 1D, a conductive thin film may remain in the "dishing" 7. If aconductive thin film remains in the "dishing" 7, this film can bringabout defective insulation in the future. This influences thereliability of the semiconductor device.

SUMMARY OF THE INVENTION

An important subject is to realize CMP capable of preventing "dishing".

To achieve the above subject, the inventors of this application havefocused attention on polishing particles contained in a polishingslurry.

Presently, the polishing particles contained in a polishing slurry arecerium oxide particles or silica particles.

A polishing slurry (to be referred to as a polishing slurry (I)hereinafter) containing cerium oxide particles has a high polishing rateof about 0.5 to 1.0 μm/min for an oxide film (SiO₂) but has a lowselection rate (SiO₂ /Si₃ N₄) of about 2 with respect to a nitride film(Si₃ N₄) as a stopper film. Even if polysilicon (Si) is used as astopper film, the selection rate (SiO₂ /Si) is about 1 to 2.

As described above, the polishing slurry (I) has a high polishing rateand a lower selection rate with respect to a stopper film. Accordingly,the polishing slurry (I) readily causes overpolishing and this mayscrape off a stopper film and enlarge "dishing".

In contrast, a polishing slurry (to be referred to as a polishing slurry(II) hereinafter) containing silica particles has a polishing rate ofabout 0.1 to 0.15 μm/min for an oxide film (SiO₂) which is lower thanthat of the polishing slurry (I). Also, when a stopper film is a nitridefilm (Si₃ N₄), the selection rate (SiO₂ /Si₃ N₄) is about 2. Even ifpolysilicon (Si) is used as a stopper film, the selection rate (SiO₂/Si) is about 1.

As described above, even the polishing slurry (II) has a low selectionrate with respect to a stopper film and therefore easily causesoverpolishing. However, since the polishing rate is lower than that ofthe polishing slurry (I), the polishing slurry (II) has the advantagethat the polishing amount can be easily controlled. By controlling thepolishing amount, the amount of overpolishing can be controlled and thisdecreases "dishing".

The polishing slurry (II) with this advantage can be preferably used ina trial manufacturing process. However, since the polishing rate is toolow, the polishing time becomes too long and this makes the slurry (II)not necessarily suitable for a mass-production process.

Furthermore, both of the polishing slurries (I) and (II) have a lowselection rate between the stopper film and the film to be polished.This makes a process 6 margin difficult to allow and hence makes theseslurries difficult to use in a mass-production process.

For the reasons as explained above, it is presently difficult to use CMPin a mass-production process.

In consideration of the above situation, it is an object of the presentinvention to provide a novel polishing slurry capable of preventing"dishing" and being preferably used in a mass-production process.

To achieve the above object, silicon nitride particles are used aspolishing particles contained in a polishing slurry in the presentinvention.

Since silicon nitride particles are used as polishing particlescontained in a polishing slurry, it is possible to obtain a polishingslurry having both a high polishing rate and a high selection rate. Thispolishing slurry hardly scrapes off the stopper film and can thereforeprevent "dishing". This makes CMP with high local planarizationfeasible.

Also, since the polishing rate is high, the polishing time can beshortened and this improves the throughput of semiconductor devices.Furthermore, a high selection rate makes a process margin easy to allow.From these points of view, a polishing slurry containing silicon nitridepolishing particles is suitable for a mass-production process.

It is also found that a polishing slurry containing silicon nitridepolishing particles can polish metals such as copper. This is partlybecause the hardness of silicon nitride is comparatively high. Thispolishing slurry is useful in the fabrication of semiconductor devicesbecause the polishing slurry can singly polish materials from siliconand silicon oxide to metals such as copper. For example, the number oftypes of polishing slurries used in the fabrication of semiconductordevices can be decreased. When the types of polishing slurries are thusdecreased, it is possible to prevent the fabrication from beingcomplicated. This also decreases the fabrication cost of semiconductordevices.

Conventionally, polishing slurries are roughly classified into twocategories: one is an oxide-based polishing slurry and the other is ametal-based polishing slurry. The oxide-based polishing slurry isrepresented by a polishing slurry containing silica polishing particles.The metal-based polishing slurry is represented by a polishing slurrycontaining alumina polishing particles. The oxide-based polishing slurryis used in polishing of silicon and silicon oxide. The metal-basedpolishing slurry is used in polishing of tungsten, copper, and aluminum.That is, polishing slurries are selected in accordance with thesubstances to be polished in the fabrication of semiconductor devices.

A polishing slurry containing silicon nitride polishing particles,however, can singly polish all of silicon, silicon oxide, tungsten,copper, and aluminum.

In addition, a polishing slurry containing silicon nitride polishingparticles can polish silicides such as glass and therefore can be usedin polishing of, e.g., liquid-crystal screens and lenses. Thisremarkably widens the range of uses of this polishing slurry.

Note that an equivalent effect can be obtained when silicon carbide orgraphite, particularly carbon graphite is used instead of siliconnitride as polishing particles.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIGS. 1A to 1D are sectional views showing trench isolation steps;

FIG. 2 is a sectional view showing a polishing apparatus used in CMPaccording to the present invention;

FIGS. 3A to 3I are sectional views showing trench isolation stepsaccording to the first embodiment of the present invention;

FIG. 4 is a graph showing the results of comparative experiments;

FIGS. 5A to 5M are sectional views showing steps of burying trencheswith polysilicon according to the second embodiment of the presentinvention;

FIGS. 6A to 6D are sectional views showing steps of burying trencheswith polysilicon;

FIGS. 7A to 7E are sectional views showing steps of forminginterconnecting lines according to the third embodiment of the presentinvention;

FIG. 8 is a graph showing the relationship between the secondaryparticle size of polishing particles and the polishing rate in apolishing slurry according to the present invention;

FIG. 9 is a perspective view of another polishing apparatus used in CMPaccording to the present invention;

FIG. 10 is a block diagram of the other polishing apparatus used in CMPaccording to the present invention; and

FIG. 11 is a block diagram of an electrolytic bath of the otherpolishing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the accompanying drawings.

FIG. 2 is a sectional view of a polishing apparatus used in CMP(Chemical Mechanical Polishing) according to the present invention.

As shown in FIG. 2, a polishing disc support 23 is arranged on a stage21 via a bearing. A polishing disc 24 is attached to the support 23. Apolishing pad 25 for polishing a wafer 20 is adhered to the polishingdisc 24. A driving shaft 26 for driving the support 23 and the polishingdisc 24 is connected to the central portions of the support 23 and thepolishing disc 24. The shaft 26 is rotated by the rotation of a rotatingbelt 28. The rotating belt 28 is rotated by a motor 27.

A wafer carrier 31 is arranged in a position opposite to the polishingpad 25. A retainer ring 29 is attached to the wafer carrier 31. Anadsorption pad 30 is attached to a hole formed in the retainer ring 29.The wafer 20 is adsorbed by the adsorption pad 30 by vacuum or water andcarried by the wafer carrier 31. The central portion of the wafercarrier 31 is connected to a driving shaft 32 for rotating the carrier31. The shaft 32 is rotated by a motor 33 via gears 34 and 35. The shaft32 is attached to a driving base 36 which is attached to a piston 38 ofa cylinder 37. The driving base 36 moves vertically when the piston 38moves vertically.

To polish the wafer 20, the wafer carrier 31 carrying the wafer 20 andthe polishing pad 24 are rotated. A polishing slurry is supplied to thepolishing pad 25 through a polishing slurry supply nozzle 39. A downforce is applied to the wafer carrier 31 by the piston 38, therebypushing the wafer 20 against the polishing pad 25 by a predeterminedpressure. The wafer 20 is polished by maintaining this state for a timerequired for polishing.

A method of fabricating a semiconductor device using CMP according tothe present invention will be described below.

Trench isolation steps will be described first as the first embodiment.Trench isolation is accomplished by burying trenches formed in a wafer(to be referred to as a silicon substrate hereinafter) with a CVD-oxidefilm (CVD-SiO₂) and planarizing the CVD-oxide film by CMP.

FIGS. 3A to 3I are sectional views showing the trench isolation steps inorder.

As shown in FIG. 3A, silicon nitride (Si₃ N₄) is deposited on a siliconsubstrate 1 to form a 70-nm thick nitride film 2. This nitride film 2serves as a polishing stopper film. Silicon dioxide (SiO₂) is depositedon the nitride film 2 to form a CVD-oxide film 3. This CVD-oxide film 3serves as a mask when trenches are formed.

In FIG. 3B, the CVD-oxide film 3 is coated with a photoresist to form aphotoresist layer 4.

In FIG. 3C, windows corresponding to a trench formation pattern areformed in the photoresist layer 4 by photolithography.

In FIG. 3D, the CVD-oxide film 3 and the nitride film 2 are etched byRIE by using the photoresist layer 4 as a mask. Thereafter, thephotoresist layer 4 is removed.

In FIG. 3E, the CVD-oxide film 3 and the silicon substrate 1 are etchedby RIE. As a consequence, trenches 5 are formed in the silicon substrate1 due to the difference between the etching rates of the CVD-oxide film3 and the silicon substrate 1. Subsequently, a damage layer formed byRIE on the surface in the trenches 5 to which the silicon substrate 1 isexposed and reaction products formed simultaneously with RIE areremoved. This is done by wet processing (wet etching).

In FIG. 3F, silicon dioxide (SiO₂) is deposited on the structure shownin FIG. 3E to form a CVD-oxide film 6. This CVD-oxide film 6 buries thetrenches 5 and at the same time covers the surface of the siliconsubstrate 1 on which the trenches 5 are formed.

In FIG. 3G, CMP is performed by the polishing apparatus shown in FIG. 2by using the CVD-oxide film 6 and the nitride film 2 as stopper films,thereby planarizing the surface of the structure shown in FIG. 3F.

In this CMP step, a novel polishing slurry according to the presentinvention is used. One example of the polishing slurry according to thepresent invention is prepared by dispersing silicon nitride particles aspolishing particles in nitric acid as a solvent. The polishing particlesact on the CVD-oxide film 6 and mechanically polish the CVD-oxide film6. The particle size of the silicon nitride particles themselves, i.e.,the particle size of primary particles is preferably 0.01 to 1000 nm. Aparticle size exceeding 1000 nm is unpreferable because the mechanicalpolishing properties become too strong and the chemical polishingproperties become extremely weak. On the other hand, if the particlesize is smaller than 0.01 nm, the mechanical polishing properties becomeweak to make well-balanced polishing impossible. The particle size ofthe primary particles is particularly preferably 10 to 40 nm. If theparticle size of the primary particles is 10 to 40 nm, well-balancedpolishing is possible.

The silicon nitride particles can also be colloidally dispersed in asolvent. When the silicon nitride particles are colloidally dispersed ina solvent, the silicon nitride particles are readily evenly dispersed inthe solvent. The particle size of the colloidal silicon nitrideparticles, i.e., the particle size of secondary particles is preferably60 to 300 nm, and particularly preferably 60 to 100 nm. The particlesize of the secondary particles can be measured by using a centrifugalprecipitation method capable of measuring particle sizes of 0.01 μm ormore.

To improve the dispersibility of polishing particles, it is alsopossible to further mix a dispersant such as a surfactant in a polishingslurry, in addition to colloidally dispersing the polishing particles ina solvent.

The viscosity of the polishing slurry is preferably 1 to 10 cp. If theviscosity is low, it becomes difficult to evenly disperse siliconnitride particles, i.e., polishing particles, in a solvent. Whenpolishing particles are evenly dispersed in a solvent, the planarity ofthe polishing surface can be easily improved. If the viscosity is toohigh, the mechanical polishing properties become strong. When themechanical polishing properties become too strong, the warpage of awafer or the uniformity of the thickness of a deposited film has a largeeffect on the planarity after CMP.

The polishing temperature is preferably 20 to 70° C. This is so becauseif the temperature is too high, the chemical action becomes too strong.

In this first embodiment, the polishing conditions are that the rotatingspeed of the polishing disc 24 is 100 rpm, the rotating speed of thewafer carrier 31 is 100 rpm, the down force applied to the wafer carrier31 is 400 g/cm², and the temperature of the polishing disc 24 is 25 to30° C. The polishing slurry used consists of nitric acid as a solventand silicon nitride particles as polishing particles and has a viscosityof 2 cp. This polishing slurry was supplied to the polishing pad at aflow rate of 300 cc/min to perform CMP on a 6" wafer (silicon substrate1).

FIG. 3G shows the section after the polishing.

As shown in FIG. 3G, when the CVD-oxide film 6 is subjected to CMP byusing the polishing slurry containing the silicon nitride particles, aprocessed shape having a small "dishing" and a high local planarizationis obtained.

Subsequently, the nitride film 2 is etched away as shown in FIG. 3H.

In FIG. 3I, finish polishing is so performed that the surface of thesilicon substrate 1 is flush with the surface of the CVD-oxide film 6.This completes trench isolation having a good processed shape withalmost no "dishing" on the silicon substrate 1 and the CVD-oxide film 6buried in the trenches 5.

FIG. 4 shows the results of experiments comparing the polishing rates,the selection rates, and the planarities of the polishing surfacesobtained by different polishing particles. Note that the polishingconditions of the comparative experiments are the same as the conditionsexplained with reference to FIG. 3G.

As shown in FIG. 4, a silicon dioxide film is polished by using apolishing slurry (IV) containing silicon nitride (Si₃ N₄) particles. Thepolishing rate is 700 to 1000 nm/min. This polishing rate compares to orexceeds a conventionally highest polishing rate of 500 to 800 nm/min ofa polishing slurry (I) containing cerium oxide (CeO) particles, and ishighest among other comparative examples.

Also, a silicon dioxide film is polished with the polishing slurry (IV)by using a silicon nitride film as a stopper film. The selection rate(SiO₂ /Si₃ N₄) is 10 to 20. This selection rate far exceeds aconventionally maximum selection rate of 2 to 3 of a polishing slurry(II) containing fumed silica (fumed SiO₂), and is maximum among othercomparative examples.

Furthermore, a silicon dioxide film is polished with the polishingslurry (IV) by using a silicon nitride film as a stopper film. Theplanarity of the polished surface is 2 to 5%. This planarity is betterthan a conventionally most accurate planarity of 10% of a polishingslurry (III) containing alumina (Al₂ O₃) particles, and is most accurateamong other comparative examples.

In the first embodiment as described above, it is possible by using thepolishing slurry (IV) containing silicon nitride particles tosimultaneously achieve high-speed polishing, a high selection rate withrespect to the stopper film, and a highly accurate planarity.

To maintain the selection rate with respect to the stopper film and theplanarity on the respective high levels, it is preferable to form thestopper film by using the same material, silicon nitride (Si₃ N₄), asthe polishing particles of the polishing slurry (IV) as in the firstembodiment.

As the second embodiment, steps of burying trenches with polysiliconwill be described below. These steps can be used to form, e.g., a memorycell of a DRAM.

FIGS. 5A to 5M are sectional views showing the steps of burying trencheswith polysilicon in the order of steps.

As shown in FIG. 5A, the surface of a silicon substrate 1 is thermallyoxidized to form a buffer oxide film (SiO₂) 8 about 10 to 50 nm thick.

In FIG. 5B, silicon nitride (Si₃ N₄) is deposited on the buffer oxidefilm 8 to form a 70-nm thick nitride film 2. This nitride film 2 servesas a polishing stopper film and an oxidation barrier film when LOCOS isperformed.

In FIG. 5C, silicon dioxide (SiO₂) is deposited on the nitride film 2 toform a CVD-oxide film 3. This CVD-oxide film 3 serves as a mask whentrenches are formed.

In FIG. 5D, the CVD-oxide film 3 is coated with a photoresist to form aphotoresist layer 9. Subsequently, windows corresponding to a trenchformation pattern are formed in the photoresist layer 9 byphotolithography.

In FIG. 5E, the CVD-oxide film 3, the nitride film 2, and the bufferoxide film 8 are etched by RIE by using the photoresist layer 9 as amask, thereby exposing the surface of the silicon substrate 1.Thereafter, the photoresist layer 9 is removed.

In FIG. 5F, the CVD-oxide film 3 and the silicon substrate 1 are etchedby RIE. As a consequence, trenches 10 are formed in the siliconsubstrate 1 due to the difference between the etching rates of theCVD-oxide film 3 and the silicon substrate 1. A damage layer formed byRIE on the silicon substrate 1 exposed to the trenches 10 and reactionproducts formed by RIE are removed. This is done by wet processing (wetetching). Subsequently, the surface of the silicon substrate 1 exposedinside the trenches 10 is thermally oxidized to form an oxide film(SiO₂) 11.

In FIG. 5G, silicon (Si) is deposited on the structure shown in FIG. 5Fby using LPCVD, forming a polysilicon film 12. This polysilicon film 12buries the trenches 10 and at the same time covers the surface of thesilicon substrate 1 on which the trenches 10 are formed.

In FIG. 5H, the polishing apparatus shown in FIG. 2 is used to performCMP for the polishing film 12 by using the CVD-oxide film 3 as a stopperfilm, thereby planarizing the surface of the structure shown in FIG. 5G(the first CMP step). This first CMP step is performed by using apolishing slurry, analogous to the polishing slurry explained in thefirst embodiment, which is prepared by dispersing silicon nitrideparticles as polishing particles in nitric acid as a solvent.Consequently, dishing formed on the exposed surface of the polysiliconfilm 12 can be decreased.

In FIG. 5I, the CVD-oxide film 3 is etched away by using an etchingsolution containing hydrofluoric acid (HF). In the structure from whichthe CVD-oxide film 3 is thus removed, the end portions of thepolysilicon film 12 protrude from the surface of the nitride film 2.

In FIG. 5J, the polishing apparatus shown in FIG. 2 is used to performCMP for the polysilicon film 12 by using the nitride film 2 as a stopperfilm, thereby planarizing the surface of the structure shown in FIG. 5I(the second CMP step). In this second CMP step, the same polishingslurry as used in the first CMP step is used. Consequently, dishingformed on the exposed surface of the polysilicon film 12 is decreased.Also, the edges of the nitride film 2 are abraded little.

In FIG. 5K, the structure shown in FIG. 5J is coated with a photoresistto form a photoresist layer 13. A portion of the photoresist layer 13which covers a prospective element isolation region is removed byphotolithography, leaving only a portion of the photoresist layer 13which covers a prospective element region.

In FIG. 5L, the surface of the silicon substrate 1 is exposed byremoving the nitride film 2 and the buffer oxide film 8 by using thephotoresist layer 13 as a mask. Thereafter, the photoresist layer 13 isremoved.

In FIG. 5M, the exposed surface of the silicon substrate 1 and thesurface of the polysilicon film 12 are thermally oxidized by using thenitride film 2 as an oxidation barrier film, thereby forming a fieldoxide film (SiO₂) 14 on the surface of the structure shown in FIG. 5L(the LOCOS step). In this LOCOS step, a bird's beak is formed along theedges of the nitride film 2. However, this bird's beak is small becausethe abrasion amount of the edges of the nitride film 2 is small. Abird's beak reduces the area of the element region and has an influenceon the characteristics of a semiconductor device.

FIGS. 6A to 6D illustrate an example in which the fabrication methodaccording to the second embodiment is performed by using a conventionalpolishing slurry. In FIGS. 6A to 6D, the same reference numerals as inFIGS. 5A to 5M denote the same parts.

FIG. 6A corresponds to the step shown in FIG. 5J.

As shown in FIG. 6A, the polishing apparatus shown in FIG. 2 is used toperform CMP for a polysilicon film 12 by using a nitride film 2 as astopper film, thereby planarizing the surface of the structure shown inFIG. 5I. This CMP is done by using a polishing slurry prepared bydispersing silica particles as polishing particles in nitric acid as asolvent. Consequently, dishing 7 is formed on the exposed surface of thepolysilicon film 12. Also, the edges of the nitride film 2 are scrapedoff and abraded.

FIG. 6B corresponds to the step shown in FIG. 5K.

FIG. 6C corresponds to the step shown in FIG. 5L.

FIG. 6D corresponds to the step shown in FIG. 5M.

As shown in FIG. 6D, the exposed surface of a silicon substrate 1 andthe surface of the polysilicon film 12 are thermally oxidized by usingthe nitride film 2 as an oxidation barrier film, thereby forming a fieldoxide film (SiO₂) 14 on the surface of the structure shown in FIG. 6C.Since the edges of the nitride film 2 are largely abraded, a bird's beaklargely extends in the interface between the nitride film 2 and thesilicon substrate 1. This is because the abraded edges of the nitridefilm 2 easily warp. The bird's beak extending toward the element regionreduces the area of the element region. In a certain MOSFET, the widthof the element region determines the gate width of the MOSFET. In aMOSFET of this sort, the gate width is narrowed by narrowing of thewidth of the element region. The gate width of a MOSFET determines thedrivability of the MOSFET. Accordingly, if the gate width greatlydeviates from the design value, the characteristics of the semiconductordevice are affected.

In contrast, in the fabrication method according to the secondembodiment, the edges of the nitride film 2 are abraded little and hencedo not easily warp. Accordingly, the bird's beak does not easily extendand the gate width does not largely deviate from the design value.Consequently, the characteristics of the semiconductor device are noteasily affected.

Also, in the fabrication method according to the second embodiment, theconversion difference between the LOCOS patterns is small because thebird's beak is small. Accordingly, the method is suitable for fineelement region patterns.

As the third embodiment, steps of forming an interconnecting pattern bytrenches, burying the trenches with a conductor, and forminginterconnecting lines will be described below. This interconnecting lineformation method is used for high-packing-density semiconductor devicesand generally called damascene process.

FIGS. 7A to 7E are sectional views showing the steps of forminginterconnecting lines in order.

As shown in FIG. 7A, a CVD-oxide film (SiO₂) 3 and a plasma oxide film(SiO₂) 15 formed by plasma CVD are formed in this order on a siliconsubstrate 1. The CVD-oxide film 3 and the plasma oxide film 15 areinsulating interlayers.

In FIG. 7B, the plasma oxide film 15 is so patterned as to form trenches17 corresponding to an interconnecting pattern.

In FIG. 7C, copper (Cu) is deposited on the structure shown in FIG. 7Bto form a copper film 16. This copper film 16 buries the trenches 17 andat the same time covers the surface of the plasma oxide film 15 on whichthe trenches 17 are formed.

In FIG. 7D, the polishing apparatus shown in FIG. 2 is used to performCMP for the copper film 16 by using the plasma oxide film 15 as astopper film, thereby planarizing the surface of the structure shown inFIG. 7C. This CMP step is done by using a polishing slurry, similar tothe polishing slurry explained in the first embodiment, which isprepared by dispersing silicon nitride particles as polishing particlesin nitric acid as a solvent. As a consequence, dishing formed on theexposed surface of the copper film 16 is decreased. By this CMP, copperis buried only in the trenches 17 to complete buried interconnectinglines in the first layer.

Subsequently, as shown in FIG. 7E, silicon dioxide (SiO₂) is depositedon the structure shown in FIG. 7D by using plasma CVD, forming a plasmaoxide film 18. Since the surface of the structure shown in FIG. 7D isaccurately planarized, the plasma oxide film 18 is easily formed.

Also, forming buried interconnecting lines using the polishing slurryaccording to the present invention facilitates the formation of buriedinterconnecting lines in the second and third layers (not shown).

The secondary particle size dependence of the polishing rate will bedescribed below.

FIG. 8 is a graph showing the relationship between the secondaryparticle size of polishing particles and the polishing rate. Thissecondary particle size dependence of the polishing rate was obtained bypolishing polysilicon with a polishing slurry prepared by dispersingsilicon nitride particles as polishing particles in nitric acid as asolvent.

Referring to FIG. 8, the ordinate indicates the polishing rate and theabscissa indicates the secondary particle size.

As shown in FIG. 8, when the secondary particle size is about 50 nm, thepolishing rate is 41.2 nm/min. However, when the secondary particle sizeexceeds about 60 nm, the polishing rate greatly increases to 810.8nm/min. When the secondary particle size is about 200 to 260 rnm, thepolishing rate is 1108.4 nm/min.

As described above, a polishing slurry containing silicon nitridepolishing particles has a tendency to critically increase its polishingrate when the secondary particle size of the polishing particles exceedsabout 60 rnm.

When the particle size of polishing particles is small, polishingproceeds mainly due to the action of chemical polishing. As the particlesize of polishing particles increases, the action of mechanicalpolishing becomes strong in polishing. It is estimated that the actionof mechanical polishing is particularly significant in a polishingslurry containing silicon nitride polishing particles when the secondaryparticle size is about 60 nm.

The secondary particle size is preferably as large as possible. However,if the secondary particle size is too large, a number of flaws may beformed on the polished surface. If a conductor such as a metal entersthese flaws, this may cause a short circuit. Therefore, the number offlaws is preferably as small as possible. For this reason, it ispreferable that the secondary particle size do not exceed 300 nm. Inaddition, the particle size of secondary particles is preferablyminimized as long as the polishing rate does not decrease. From thesepoints of view, the particle size of secondary particles is particularlypreferably about 60 to 100 nm.

As a solvent used in the polishing slurry of the present invention, itis possible to use an emulsifying agent, water, a surfactant, fats andoils, an adhesive, and ionized water, in addition to nitric acid. Also,an acidic solvent is primarily used as a solvent. One representativeexample is nitric acid. Examples of an alkaline solvent are ammonia,amines such as piperazine, and inorganic alkalis such as potassiumhydroxide and sodium hydroxide. Any of these alkaline solvents can alsobe used as a solvent.

It is also possible to prepare a diluted polishing slurry by adding adispersant, e.g., ionized water, to the polishing slurry according tothe present invention. When a polishing slurry like this is used, notonly the polishing slurry contributes to polishing but there is also anauxiliary polishing action by the dispersant. A solvent of a polishingslurry also has a dispersion effect.

Examples of the dispersant are an emulsifying agent, water, asurfactant, fats and oils, an adhesive, and ionized water (alkaliionized water and acidic ionized water).

If a polishing slurry added with a dispersant is left to stand, however,the dispersant sometimes reacts with particularly the solvent of thepolishing slurry to deteriorate the polishing slurry. To prevent thisdeterioration of a polishing slurry, it is possible to simultaneouslyadd a polishing slurry and a dispersant to the process point of apolishing pad when CMP is performed. Especially when a dispersant isalkali ionized water, it is preferable to produce alkali ionized waterand at the same time supply the produced water together with a polishingslurry to the process point of a polishing pad. This is so becausealkali ionized water cannot be stored for long time periods.

FIG. 9 is an enlarged perspective view of an essential part of a CMPapparatus (Polisher) according to a first embodiment of the invention,showing a wafer carrier 31, a polishing disc 24 and its peripheralelements. This CMP apparatus has basically the same structure as theFIG. 2 apparatus. In the CMP apparatus of the fourth embodiment, thepolishing disk 24 has a support unit (not shown) provided at a lowerportion thereof, and a driving shaft (not shown) is provided at a centerportion of the polishing disk 24. The polishing disc 24 rotates togetherwith the driving shaft when a motor (not shown) rotates. A polishing pad25 formed of polyurethane foam or polyurethane nonwoven fabric isattached to the upper surface of the polishing disc 24. A semiconductorwafer (not shown) is held by the wafer carrier 31 by a vacuum force,etc. such that it is opposed to the polishing pad 24. A driving shaft 32is provided on a center portion of the wafer carrier 31 for rotating thecarrier 31 in accordance with the rotation of a motor (not shown).Further, the semiconductor wafer held by the wafer carrier 31 is pressedagainst the polishing pad 24 and released therefrom in accordance withthe movement of the driving shaft 32.

In the invention, at the time of polishing a semiconductor wafer, apolishing slurry which contains silicon nitride (Si₃ N₄) particles issupplied from a polishing tank through a polishing-slurry supply pipe 39to the polishing pad 25, and at the same time ionized water is suppliedfrom an ionized-water supply pipe 40 to the polishing pad 25. To thisend, the polishing-slurry supply pipe 39 and the ionized-water supplypipe 40 have their nozzles located above the polishing pad 25 and in thevicinity of the wafer carrier 31 which holds the semiconductor wafer.The polishing slurry and ionized water are supplied to the working areaof the polishing pad 25 and mixed with each other by means of the supplypipes 39 and 40. The supply pipes 39 and 40 are movable over thepolishing pad 24.

At the time of polishing, the semiconductor wafer is pressed against thepolishing pad 25 on the polishing disc 24 with a down force of 50 to 500g/cm while the pad 25 and the disc 24 rotate at 20 to 200 rpm.

Ionized water supplied from the ionized-water supply pipe 40 can beclassified into alkaline ionized water and acidic ionized water. Ionizedwater of a desired pH is created by electrolyzing, at a low voltage,deionized water which contains no electrolyte, i.e. no metal impurity,in an electrolytic bath with a solid electrolyte contained therein.Where alkaline ionized water is used and the polishing rate is changedduring polishing, the rate can be increased in a stable manner byincreasing the pH value of alkaline ionized water, and can be reduced ina stable manner by reducing the pH value of alkaline ionized water. Onthe other hand, in the case of using acidic ionized water, the polishingrate can be increased in a stable manner by reducing the pH value ofacidic ionized water, and can be reduced in a stable manner byincreasing the pH value of acidic ionized water.

FIG. 10 is a sectional view, showing a CMP apparatus according to afifth embodiment of the invention, and specifically showing a wafercarrier, a polishing disc, a waste water mechanism and an electrolyticbath for supplying ionized water. This CMP apparatus has the same basicstructure as that shown in FIG. 2. A polishing pad 25 formed ofpolyurethane nonwoven fabric, etc. for polishing a semiconductor waferis attached to the upper surface of a polishing disc 24. A semiconductorwafer (not shown) is held by a wafer carrier 31 by a vacuum or mater,etc. such that it is opposed to the polishing pad 25. A driving shaft 32is provided on a center portion of the wafer carrier 31 for rotating thecarrier 31 in accordance with the rotation of a motor (not shown).Further, the semiconductor wafer held by the wafer carrier 31 is pressedagainst the polishing pad 25 and released therefrom in accordance withthe movement of the driving shaft 32.

At the time of polishing the semiconductor wafer, a polishing slurrycontaining polishing particles such as silicon nitride particles issupplied from a polishing slorry tank 41 through a polishing-slurrysupply pipe 39 to the polishing pad 25, and at the same time ionizedwater is supplied from an ionized-water supply pipe 40 to the polishingpad 25. To this end, the polishing-slurry supply pipe 39 and theionized-water supply pipe 40 have their nozzles located above thepolishing pad 25 and in the vicinity of the wafer carrier 31 which holdsthe semiconductor wafer. The supply pipes 39 and 40 are disposed to bemovable over the polishing pad 25, thereby supplying the polishingslurry and ionized water to the working area of the polishing pad 25 andpermitting them to be mixed with each other.

The rotary polishing disc 24 is received in an envelope 43 so as toprevent the wasted polishing slurry and ionized water having been usedfor polishing from scattering to the outside. An exhaust port 44 isformed in the bottom of the envelope 43 for exhausting the wasted water.A wasted water pipe 45 is connected to the exhaust port 44. A one-wayvalve 46 is voluntarily provided across the waste pipe 45 for preventingreverse flow of the wasted water. Further, a sediment bath 47 isvoluntarily provided for separating polishing particles having been usedfor polishing from the wasted water.

The polishing-slurry supply pipe 39 is connected to the polishing slurrytank 41, while the ionized-water supply pipe 40 is connected to anelectrolytic bath 42. An ionized-water exhaust pipe 48 is connected tothe wasted water pipe 45 lead from the envelope 43. A one-way valve maybe provided across the ionized-water exhaust pipe 48, too.

The above-described pipe structure enables the ionized water exhaustedfrom the electrolytic bath 42 to be mixed with the ionized watercontained in the wasted water, thereby neutralizing them. Thus, thewasted pipe 45 and the ionized-water exhaust pipe 48 connected theretoconstitute a neutralization mechanism. In general, when in theelectrolytic bath, alkaline (or acidic) ionized water is created, acidic(or alkaline) ionized water of the same amount is also created.Therefore, in the case of performing polishing using alkaline ionizedwater, acidic ionized water created simultaneously and not necessary forpolishing is collected through the ionized-water exhaust pipe 48 to thewasted-water pipe 45 to neutralize the wasted water created duringpolishing.

FIG. 11 is a sectional view of a CMP apparatus according to a sixthembodiment of the invention, showing an electrolytic bath. As is shownin FIG. 11, an electrolytic bath 42 has a cathode chamber 51 and ananode chamber 52. The cathode chamber 51 contains a cathode electrode53, while the anode chamber 52 contains an anode electrode 54. Theseelectrodes 53 and 54 are formed of platina or titanium. The cathode andanode chambers 51 and 52 are partitioned by a porous barrier membrane 55for efficiently separating negative-ionized water 63 created in thecathode chamber 51, from positive-ionized water 64 created in the anodechamber 52. The cathode electrode 53 in the electrolytic bath 42 isconnected to the negative electrode 57 of a battery 56, and the anodeelectrode 54 to the positive electrode 58 of the battery 56.

In the electrolytic bath 42, a diluent electrolyte solution 59, with asupporting electrolyte (e.g. ammonium chloride) contained therein ismixed with deionized water, and a power voltage is applied thereto fromthe battery 56, thereby electrolyzing the deionized water.Negative-ionized water 63 created on the side of the cathode electrode53 as a result of electrolyzation is alkaline ionized water, whilepositive-ionized water 64 created on the side of the anode electrode 54is acidic ionized water. Moreover, if deionized water is electrolyzed inthe bath 42 using oxalic acid as the supporting electrolyte, bothnegative-ionized water created on the side of the cathode andpositive-ionized water created on the side of the anode exhibit acidicproperties. The negative-ionized water 63 in the cathode chamber 51 issupplied to the outside through a negative-ionized water supply pipe 61,and the positive-ionized water in the anode chamber 52 is supplied tothe outside through a positive-ionized water supply pipe 62.

Since alkaline ionized water is usually created in the cathode chamber51, the negative-ionized water supply pipe 61 connected to theelectrolytic bath 42 is used, in the sixth embodiment, as theionized-water supply pipe 40 for supplying alkaline ionized water to thepolishing pad 25, when polishing is performed using alkaline ionizedwater. In this case, acidic ionized water created in the anode chamber52 is not necessary and hence exhausted. Accordingly, thepositive-ionized water supply pipe 62 is used as the ionized waterexhaust pipe 48 for exhausting ionized water, and connected to thewasted water pipe 45. On the other hand, if polishing is performed usingacidic ionized water, the positive-ionized water supply pipe 62connected to the electrolytic bath 42 is used as the ionized-watersupply pipe 40 for supplying acidic ionized water to the polishing pad25. In this case, alkaline ionized water created in the cathode chamber51 is not necessary and hence exhausted. Thus, the negative-ionizedwater supply pipe 61 is used as the ionized water exhaust pipe 48 forexhausting ionized water, and connected to the wasted water pipe 45.(See FIG. 10)

Ionized water can be classified into alkaline ionized water and acidicionized water. Ionized water of a desired pH is created byelectrolyzing, at a low voltage, deionized water which contains noelectrolyte, i.e. no metal impurity, in an electrolytic bath with asolid electrolyte contained therein. Where in the case of using alkalineionized water, the polishing rate is changed during polishing, it can beincreased in a stable manner by increasing the pH value of alkalineionized water, and can be reduced in a stable manner by reducing the pHvalue of alkaline ionized water. On the other hand, in the case of usingacidic ionized water, the polishing rate can be increased in a stablemanner by reducing the pH value of acidic ionized water, and can bereduced in a stable manner by increasing the pH value of acidic ionizedwater.

Whether alkaline ionized water or acidic ionized water is used dependsupon the kind of a film deposited on a semiconductor wafer.

Ionized water can electrically stabilize the surface of the filmdeposited on the wafer obtained after polishing. Acidic ionized water issuitable for a deposit film made of a metal with a high melting point,such as Al, Cu, W, etc. Such a deposit film has its surface oxidized byacidic ionized water after polishing, with the result that the potentialof the surface is stabilized.

Alkaline ionized water or acidic water is suitable for a deposit filmmade of silicon oxide (SiO₂), silicon nitride (Si₃ N₄), polysilicon ormonocrystalline silicon. Alkaline ionized water can electricallystabilize the surface of such a deposit film. Alkaline ionized watershould be used case where a SiO₂ film is polished, and case where apolysilicon film is polished. On the other hand, acidic ionized watershould be used case where a Cu film is polished.

Conditions such as the polishing rate, the degree of stabilization of adeposit film, etc. depend upon the pH value of ionized water. Therefore,adjustment of the pH value of ionized water is very important to setoptimal polishing conditions. Since the pH value of ionized waterdepends upon its temperature. It can be accurately adjusted bycontrolling the temperature of ionized water. The pH value of acidicionized water increases as its temperature increases, whereas that ofalkaline ionized water reduces as its temperature increases. Further,the rate of change in pH is higher in alkaline ionized water than inacidic ionized water.

Conventionally, polishing slurries are roughly classified into twocategories: one is an oxide-based polishing slurry and the other is ametal-based polishing slurry. The oxide-based polishing slurry isrepresented by a polishing slurry containing silica polishing particles.The metal-based polishing slurry is represented by a polishing slurrycontaining alumina polishing particles. The oxide-based polishing slurryis used in polishing of silicon and silicon oxide. The metal-basedpolishing slurry is used in polishing of tungsten, copper, and aluminum.That is, polishing slurries are selected in accordance with thesubstances to be polished in the fabrication of semiconductor devices.

A polishing slurry containing silicon nitride polishing particles,however, can singly polish all of silicon, silicide silicon oxide,silicon nitride tungsten, copper, gold and aluminum.

In addition, a polishing slurry containing silicon nitride polishingparticles can polish silicides such as glass and therefore can be usedin polishing of, e.g., liquid-crystal screens and lenses. Thisremarkably widens the range of uses of this polishing slurry.

Furthermore, an equivalent effect can be obtained by using siliconcarbide or graphite, particularly carbon graphite, instead of siliconnitride, as polishing particles.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appeneded claims and their equivalent.

What is claimed is:
 1. A polishing slurry comprising:a solvent; andpolishing particles dispersed in said solvent and comprising siliconnitride,wherein said polishing particles are comprised of primaryparticles and secondary particles colloidally dispersed in said solvent.2. The slurry according to claim 1, wherein a primary particle size ofsaid polishing particles is 10 to 40 nm.
 3. The slurry according toclaim 1, wherein a secondary particle size of said polishing particlesis 60 to 300 nm.
 4. The slurry according to claim 2, wherein a secondaryparticle size of said polishing particles is 60 to 300 nm.
 5. The slurryaccording to claim 1, wherein a viscosity of said solvent is 1 to 10 cp.6. The slurry according to claim 2, wherein a viscosity of said solventis 1 to 10 cp.
 7. The slurry according to claim 3, wherein a viscosityof said solvent is 1 to 10 cp.
 8. The slurry according to claim 4,wherein a viscosity of said solvent is 1 to 10 cp.
 9. The slurryaccording to claim 1, wherein said solvent is acidic.
 10. The slurryaccording to claim 9, wherein said acidic solvent is nitric acid. 11.The slurry according to claim 2, wherein said solvent is alkaline. 12.The slurry according to claim 11, wherein said alkaline solvent is onemember selected from the group consisting of ammonia, piperazine,potassium hydroxide and sodium hydroxide.
 13. The slurry according toclaim 1, wherein said solvent comprises at least one member selectedfrom the group consisting of an emulsifying agent, water, ionized water,a surfactant and fats and oils.
 14. The slurry according to claim 1,wherein said solvent has a chemical etching effect.
 15. The slurryaccording to claim 1, further comprising:a dispersant added to saidsolvent.
 16. The slurry according to claim 15, wherein said dispersantcomprises at least one member selected from the group consisting of anemulsifying agent, water, ionized water, a surfactant and fats and oils.17. The slurry according to claim 1, wherein said slurry is used inchemical mechanical polishing.